1. Field of the Invention
The present invention relates to a process for preparing a high purity ambient temperature molten salt consisting essentially of an aluminum halide and an onium halide, and which is valuable in the field of using an ambient temperature molten salt, especially for efficiently effecting aluminum electroplating by using a plating bath comprising an ambient temperature molten salt.
2. Prior Art
It has been well known that an ambient temperature molten salt consisting of an aluminum halide and an onium halide is in a liquid state at an ambient temperature of an ordinary temperature and has high conductivity. Particularly, an ambient temperature molten salt consisting of an N,N'-dialkylimidazolium halide and an aluminum halide is greatly expected to be an electrolyte having novel characteristics which are extremely different from those of a conventional organic or inorganic electrolyte.
For example, a secondary battery using an ambient temperature molten salt comprising a 1,2,3-trialkylimidazolium halide and an alkyl halide as an electrolyte is proposed in Japanese Provisional Patent Publications No. 133669/1985 and No. 133670/1985. Also, a secondary battery using an ambient temperature molten salt comprising 1-ethyl-3-methylimidazolium chloride and aluminum trichloride as an electrolyte is proposed in Japanese Provisional Patent Publication No. 165879/1987. Further, a secondary battery using an ambient temperature molten salt comprising a 1,3-dialkylimidazolium halide and a halide of a IIIb metal of the periodic table as an electrolyte is proposed in Japanese Provisional Patent Publication No. 136180/1985.
For preparing such an ambient temperature molten salt, it has been known a solid mixing method in which an aluminum halide and an onium halide which are each in a solid state are gradually mixed in a nitrogen-substituted gloved box (for example, ELECTROCHEMISTRY, vol. 54 (3), p. 257).
Recently, a method in which an aluminum halide is reacted with an onium halide in an inert solvent having a low boiling point (Japanese Provisional Patent Publication No. 24088/1991 which corresponds to U.S. Pat. No. 5,135,825 and EP-B1-0 404 179).
However, it has been extremely difficult to prepare a high purity ambient temperature molten salt in an industrial scale easily because the chemical materials used in these methods are hygroscopic.
On the other hand, the aluminum electroplating has been carried out by using a non-aqueous plating bath, particularly an organic solvent plating bath, because aluminum has a high affinity for oxygen and a standard electrode potential thereof is poor than that of hydrogen.
As the organic solvent plating bath, there may be exemplified those in which aluminum trichloride and either LiAlH.sub.4 or LiH are dissolved in an ether, and aluminum trichloride and LiAlH.sub.4 are dissolved in tetrahydrofuran as representative examples (for example, D. E. Couch et al.; J. Electrochem., vol. 99 (6), p. 234). However, these plating baths each contain an extremely active LiAlH.sub.4 or LiH and therefore, if oxygen or water is present, LiAlH.sub.4 or LiH reacts with it to be decomposed, causing a problem that the current efficiency is lowered or that the life span of the bath is shortened. Such a plating bath also has a serious problem that the bath easily explodes or flames because an organic solvent used therein has a low flashing point.
As another example, a plating bath in which triethylaluminum and sodium fluoride are dissolved in toluene has been proposed (R. Suchentrunk; Z. WERKSTOFFTECH., vol. 12, p. 190). In this case, however, it is extremely difficult to deal with triethylaluminum having a high risk and it has been considered impossible to put such a plating bath into practical use.
Recently, a process for effecting aluminum electroplating has been proposed in Japanese Provisional Patent Publications No. 70592/1987, No. 272788/1989 and No. 272790/1989 in which a molten salt consisting of an aluminum halide and an onium halide is used as a plating bath. Such a plating bath, however, has a plating condition which easily changes in response to contamination with water, and even if the electroplating is carried out under constant conditions, a plating film having uniform quality cannot necessarily be obtained.